Prolog factorial recursion

Question

I\'m having trouble understanding the following factorial program

fact1(0,Result) :-
    Result is 1.
fact1(N,Result) :-
    N > 0,
    N1 is N-1,
    fac         


        

Answer 1

factorial(1, 1).
factorial(N, Result) :- M is N - 1,
factorial(M, NextResult), Result is NextResult * N.

Answer 2

A simple way :

 factorial(N, F):- N<2, F=1.

 factorial(N, F) :- 
     M is N-1, 
     factorial(M,T), 
     F is N*T.